4.0/4.2 head swap

[backroader]

Yes, I will post if I do the swap. I will need to do something sooner or later with the head, due to the worn guides in the one that is on there now.

My Motorcraft distributor has never given me much trouble, but it does have that lazy advance curve typical of the early 80's smog bandaids.. I will probably run it for a while, but may just swap it out for an HEI sometime...

Thanks again..

[tgreese]

The 232 high compression head is interesting. I thought maybe it's the same as the '87-ish 4.0L head, but the 4.0L has more hemispherical combustion chambers. Suspect it's a '71-ish head, and later years got lower CRs.

According to jeeptech.com, the 258 had the following compression ratios -
'71 8.5:1
'72-78 8.0:1
'79-81 8.3:1
'82-97 9.2:1

They could have changed either the head or the pistons to make these changes. Might be worth looking into, say you already had tubing headers and wanted to keep the 258 port pattern. Presumably 258 headers will not fit a 4.0L HO head ... maybe the '87-ish 4.0L head.

[backroader]

According to info on Wiki about the DJ5, they built 6000 DJ5-F models from 1977 to 1978 that came equipped with either the 232 or the 258.

And Jeg's shows photos of the closed chamber style head with the casting number I gave earlier. They say it fits from 1977 to 80 258's.

I haven't removed the valve cover on the 1978 engine I have in my truck to check the casting number, but the head on the 1981 engine I replaced had the open larger chamber it it. Both engines have dished pistons.

[tgreese]

Regarding the DJs, if you look at the Wiki page again you'll see that AMC/AM General supplied the majority of postals with the 232 from 1970-79. Not the only engine, but the majority after the AMC takeover. I think the few 258s were used because they stopped producing the 232 altogether after '78. From year to year, the main difference was which transmission they used.

Prior to 1970, Kaiser sourced the Chevrolet 153cid 4-cylinder and Powerglide for postals - they made a lot of those too.

Not sure there's a point to this... other than there's a lot of history to sort through if you want to associate a cylinder head design with a particular year or model of postal. IME detailed info about the postals is scarce.

[backroader]

The point for my purposes is I don't know the origin of the engine in my truck. I pulled it out of a 1971 Wagoneer that I had bought for parts, but the engine wasn't original to that vehicle, since the block numbers show it was built in 1978.

So it may possibly have originated in a DJ5 F, for all I know...

I don't recall what the cylinder chambers even looked like, wasn't something I was looking for, since I wasn't aware there were two different types of head chambers available. I most likely won't know for sure which type it is either until I actually remove the head. Unless I can find particular information about the head from the casting number.

I do recall doing compression tests on all three of the engines I have, and the readings were significantly higher for the engine I rebuilt, for some reason. Whether or not that even means anything...

[Srdayflyer]

back to my orig. post, i only mentioned the difference in head chambers (open/closed)was to assist in providing and alternative option, as far as pistons go i have found three types (dished) most common , (D shaped dish) fairly hard to find , and flat topped custom ordered, use to be avail thru custom vendors ( keith black) etc. if i recall they were a little pricy. dont recall compression ratio change. but the shear difference in chamber sz will give you a boost in c.r., the only true way determine would be to c.c. the heads to determine chamber sz, and use math to determine the true c.r., s far as high ratios on rebuilds everything is tight, frankly my engine 258 in my cj after rebuild was sluggish in acceleration and at idle it wasnt good till i got 2500-3000 mi on it then it ran good and c.r dropped a little bit and i always do differential compression tests because i get way more info , leaking intake/exhaust valves,how tight the combustion chamber is , ring blow by, .

[backroader]

It would make sense to me that by running the head with the closed chamber you would gain more power by raising the compression ratio.

As long as you could still run regular gas, it would be a cheap way to gain some power.

One thing I noticed after the rebuild was considerably less blow-by moisture in the oil fill cap than the other engine, so I took that as a good sign of the overall engine health. Not happy with the work done by the machine shop on my head, however...

[tgreese]

The point for my purposes is I don't know the origin of the engine in my truck. I pulled it out of a 1971 Wagoneer that I had bought for parts, but the engine wasn't original to that vehicle, since the block numbers show it was built in 1978. ...

The block is not required to match the head, and vice-versa. Some '80-ish engines had 7/16" head bolts, but the rest have 1/2" and are compatible AFAIK. You could have a '71 head on a '78 block, for example. I believe up through mid-74, the 258 head was like the earlier Rambler 199/232 six, and had a rocker shaft. Later heads used bridged rockers. Likely there are some casting numbers online somewhere. Maybe here - https://forums.off-road.com/threads/jee ... ory.76020/ If the casting number is not visible, it probably can be seen under the valve cover. It will be somewhere in the raw casting surface of the head.

[backroader]

The casting number is under the valve cover between the rockers near the center of the head.

So the valve cover needs to be removed to see it. Different than the 4.0 head, the number is outside the cover on the side.

I don't want to re-use the head on the 1981 engine that came out of the truck even though it is in okay condition for several reasons. It uses 7/16" head bolts rather than 1/2". It has the plastic valve cover. The holes are there for a bolt-on style cover, but not tapped for threads. And it also has the larger open combustion chambers.

If I discover the head I'm currently using also has the large chambers, I will be in the market for a head with the smaller chambers for sure. If it has the small chambers, it will most likely be going back to the machine shop for new guides and a valve grind. Or just a complete replacement with a rebuilt head.

[Srdayflyer]

regarding the closed chamber head, perhaps it was glossed over but i run unleaded regular with no knock or ping and run 10 degrees advance for altitude changes here in arizona so i dont have to open the hood and change timing, i want to put the holly sniper/ hyperspark combo on the cj5 so i can change f/a ratios and timing from the drivers seat,

[backroader]

After further research, I discovered that the heads produced from 1971 to 1976 with the open chambers actually had smaller chambers than the later heads with the semi-closed chambers. The listing shows 62 ccs for the older style and 74 ccs for the later style.

Curious to know why the 1971 model 258 with a 1 barrel carb produced significantly more power than all later versions, according to Jeep Tech..

[tgreese]

Change from Gross to Net HP ratings?

[tgreese]

...
I don't want to re-use the head on the 1981 engine that came out of the truck even though it is in okay condition for several reasons. It uses 7/16" head bolts rather than 1/2". ...

I recall you can use a head with 1/2" holes with 7/16" bolts if you put spacers in the bolt holes. HESCO sells spacers IIRC, or a friendly machinist could easily make them.

... It has the plastic valve cover. The holes are there for a bolt-on style cover, but not tapped for threads. ...

IIRC the heads meant for the plastic cover always had these blind holes that looked like they should have bolts. Not meant as bolt holes AFAIK.

[backroader]

Change from Gross to Net HP ratings?

They show torque at 1800 rpms as 240#, which is considerably higher than any of the later models. Would the ratings change also affect the torque output?

[backroader]

...
I don't want to re-use the head on the 1981 engine that came out of the truck even though it is in okay condition for several reasons. It uses 7/16" head bolts rather than 1/2". ...

I recall you can use a head with 1/2" holes with 7/16" bolts if you put spacers in the bolt holes. HESCO sells spacers IIRC, or a friendly machinist could easily make them.

Yes, It would require drilling out the holes in the 1981 head to 1/2".

... It has the plastic valve cover. The holes are there for a bolt-on style cover, but not tapped for threads. ...

IIRC the heads meant for the plastic cover always had these blind holes that looked like they should have bolts. Not meant as bolt holes AFAIK.

Yeah, I never checked to make sure they even matched the older style, but I'm positive I don't want any more of those plastic valve covers...

[backroader]

In some of the official AMC service manuals, they refer to the older heads with the smaller chambers as "wedge style" heads, and they call the newer semi-closed chamber heads a "quench style".

I think the theory is that the quench style are supposedly more efficient. Not sure why they would have increased the chamber size.

[tgreese]

Change from Gross to Net HP ratings?

They show torque at 1800 rpms as 240#, which is considerably higher than any of the later models. Would the ratings change also affect the torque output?

Yes. Power is torque times RPM. They change with each other.

[backroader]

The only apparent difference in the specs of the 1971 model engine versus the later models besides the hp & torque figures is the compression ratio. 8.5 to 1, as opposed to 8.0 to 1 for later models up to the later ones they produced in the 80's.

From the information I've seen, they used the same heads in the later 70's engines, so the difference must have came from a different deck height or different pistons.

[backroader]

I've never actually compared numbers between the two, but the 258 is basically a stroked 232, so I'm thinking a 4.0 must be basically a bored out 232.

[tgreese]

Pretty sure the stroke of a 232 is longer than the 4.0L. A 4.0L stroker with a 232 crank is a thing IIRC.